Rubber composition for tire tread, and pneumatic tire using same

ABSTRACT

A rubber composition for a tire tread comprising: a diene rubber; a silica having an N 2 SA of from 194 to 225 m 2 /g and a CTAB of from 180 to 210 m 2 /g; and a polysiloxane represented by the average composition formula of formula (1) and contained in an amount of from 1 to 20 mass % with respect to the quantity of silica. (A) a (B) b (C) c (D) d  (R 1 ) 3 SiO (4-2a-b-c-d-e)/2 . Also provided is a pneumatic tire having tire treads formed using the rubber composition.

TECHNICAL FIELD

The present technology relates to a rubber composition for a tire treadand a pneumatic tire using the same.

BACKGROUND

Conventionally, there has been a demand to reduce tire rollingresistance from the perspective of low fuel consumption at the time ofvehicle traveling. In addition, there has been a demand for improvementsin wet performance from the perspective of safety. A known method ofachieving this is a method of establishing both low rolling resistanceand wet performance by adding silica to a rubber component constitutingthe tread part of a tire.

However, silica has low affinity with rubber components, and thecohesiveness of silica components is high, so even if silica is simplyadded to the rubber component, the silica is not dispersed, and whichleads to the problem that the effect of reducing the rolling resistanceor the effect of improving the wet performance cannot be sufficientlyachieved.

Under such circumstances, the present applicant proposed a rubbercomposition containing a specific silica (Japanese Unexamined PatentApplication Publication No. 2012-121936).

In addition, there is also a demand for there to be little crosslinking(rubber burning) in the rubber composition for a tire tread at thestorage stage or the stage before the vulcanization process. That is,there is a demand for excellent processability (for example, for theviscosity of the rubber composition to be appropriate and for the scorchresistance and extrudability to be excellent; same hereafter).

In recent years, environmental issues and resource problems have led toa demand for even lower fuel consumption in vehicles, which in turn hasled to a demand for further improvements in the low rolling resistanceof tires. In addition, in step with improvements in the required safetylevel, there has also been a demand for further improvements in wetperformance. Due to such reasons, the present inventors assessed thatthere is room for improvement in tire performance such as the lowrolling resistance or wet performance of a rubber composition containinga mercaptosilane that can react with silica.

When the present inventors researched rubber compositions containingsilica used in the rubber composition described in Japanese UnexaminedPatent Application Publication No. 2012-121936 and a conventionalsulfur-containing silane coupling agent, it became clear that the lowrolling resistance, wet performance, and processability do not satisfythe currently required levels.

SUMMARY

The present technology provides a rubber composition for a tire treadhaving excellent wet performance and low rolling resistance when formedinto a tire as well as excellent processability.

A rubber composition for a tire tread containing a diene rubber, silica,and a sulfur-containing silane coupling agent;

a nitrogen adsorption specific surface area (N₂SA) of the silica beingfrom 194 to 240 m²/g; a CTAB specific surface area (CTAB) of the silicabeing from 180 to 215 m²/g; a content of the silica being from 60 to 200parts by mass per 100 parts by mass of the diene rubber;

the sulfur-containing silane coupling agent being a polysiloxanerepresented by the following formula (1); and a content of thesulfur-containing silane coupling agent being from 1 to 20 mass % withrespect to the content of the silica; is a rubber composition for a tiretread having excellent wet performance and low rolling resistance aswell as excellent processability.(A)_(a)(B)_(b)(C)_(c)(D)_(d)(R¹)_(e)SiO_((4-2a-b-c-d-e)/2)  (1)[Formula (1) is an average composition formula, wherein A is a divalentorganic group containing a sulfide group; B is a monovalent hydrocarbongroup having from 5 to 10 carbon atoms; C is a hydrolyzable group; D isan organic group containing a mercapto group; R¹ is a monovalenthydrocarbon group having from 1 to 4 carbon atoms; and a to e satisfythe relational expressions 0≦a<1, 0≦b<1, 0<c<3, 0<d<1, 0≦e<2, and0<2a+b+c+d+e<4. However, at least one of a and b is not 0.]

That is, the present technology provides the following rubbercomposition for a tire and a pneumatic tire using the same.

1. A rubber composition for a tire tread containing a diene rubber,silica, and a sulfur-containing silane coupling agent;

a nitrogen adsorption specific surface area (N₂SA) of the silica beingfrom 194 to 240 m²/g; a CTAB specific surface area (CTAB) of the silicabeing from 180 to 215 m²/g; a content of the silica being from 60 to 200parts by mass per 100 parts by mass of the diene rubber;

the sulfur-containing silane coupling agent being a polysiloxanerepresented by the following formula (1); and a content of thesulfur-containing silane coupling agent being from 1 to 20 mass % withrespect to the content of the silica.(A)_(a)(B)_(b)(C)_(c)(D)_(d)(R¹)_(e)SiO_((4-2a-b-c-d-e)/2)  (1)[Formula (1) is an average composition formula, wherein A is a divalentorganic group containing a sulfide group; B is a monovalent hydrocarbongroup having from 5 to 10 carbon atoms; C is a hydrolyzable group; D isan organic group containing a mercapto group; R¹ is a monovalenthydrocarbon group having from 1 to 4 carbon atoms; and a to e satisfythe relational expressions 0≦a<1, 0≦b<1, 0<c<3, 0<d<1, 0≦e<2, and0<2a+b+c+d+e<4. However, at least one of a and b is not 0.]

2. The rubber composition for a tire tread according to 1 describedabove, wherein a DBP absorption number of the silica is at least 190ml/100 g, and a ratio of the nitrogen adsorption specific surface areaand the CTAB specific surface area (nitrogen adsorption specific surfacearea/CTAB specific surface area) is from 0.9 to 1.4.

3. The rubber composition for a tire tread according to 1 or 2 describedabove further containing a terpene resin, wherein an amount of theterpene resin is from 1 to 30 parts by mass per 100 parts by mass of thediene rubber, and the terpene resin is an aromatic modified terpeneresin having a softening point of from 60 to 150° C.

4. The rubber composition for a tire tread according to one of 1 to 3described above, wherein b is greater than 0 in formula (1).

5. A pneumatic tire comprising tire treads formed using the rubbercomposition for a tire tread described in any one of 1 to 4 above.

According to the present technology, it is possible to provide a rubbercomposition for a tire tread having excellent wet performance and lowrolling resistance when formed into a tire as well as excellentprocessability, and a pneumatic tire having tire treads formed using therubber composition.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a partial cross-sectional schematic view of a tire thatrepresents one embodiment of the pneumatic tire of the presenttechnology.

DETAILED DESCRIPTION

The present technology is described in detail below.

The rubber composition for a tire tread according to the presenttechnology is:

a rubber composition for a tire tread containing a diene rubber, silica,and a sulfur-containing silane coupling agent;

a nitrogen adsorption specific surface area (N₂SA) of the silica beingfrom 194 to 240 m²/g; a CTAB specific surface area (CTAB) of the silicabeing from 180 to 215 m²/g; a content of the silica being from 60 to 200parts by mass per 100 parts by mass of the diene rubber;

the sulfur-containing silane coupling agent being a polysiloxanerepresented by the following formula (1); and a content of thesulfur-containing silane coupling agent being from 1 to 20 mass % withrespect to the content of the silica.(A)_(a)(B)_(b)(C)_(c)(D)_(d)(R¹)_(e)SiO_((4-2a-b-c-d-e)/2)  (1)[Formula (1) is an average composition formula, wherein A is a divalentorganic group containing a sulfide group; B is a monovalent hydrocarbongroup having from 5 to 10 carbon atoms; C is a hydrolyzable group; D isan organic group containing a mercapto group; R¹ is a monovalenthydrocarbon group having from 1 to 4 carbon atoms; and a to e satisfythe relational expressions 0≦a<1, 0≦b<1, 0<c<3, 0<d<1, 0≦e<2, and0<2a+b+c+d+e<4. However, at least one of a and b is not 0.]

The rubber composition for a tire tread according to the presenttechnology is also referred to as the “composition of the presenttechnology” hereinafter. In addition, the polysiloxane represented byformula (1) is also referred to as the “polysiloxane represented by theaverage composition formula of formula (1)”.

The composition of the present technology has excellent wet performance,low rolling resistance, and processability as a result of using thepolysiloxane represented by the average composition formula of formula(1) as a sulfur-containing silane coupling agent in a rubber compositioncontaining a diene rubber and a specific silica.

In the present technology, the polysiloxane represented by the averagecomposition formula of formula (1) makes it possible to blend silicainto the rubber composition in a large quantity or to sufficientlydisperse a large amount of silica into the rubber composition.

The present inventors believe that the rubber composition for a tireaccording to the present technology achieves the effects described aboveas follows.

The skeleton of the sulfur-containing silane coupling agent contained inthe rubber composition for a tire according to the present technology[polysiloxane represented by the average composition formula of formula(1)] is a siloxane structure. In addition, when the sulfur-containingsilane coupling agent has a monovalent hydrocarbon group having from 5to 10 carbon atoms represented by B, B may function as an effectiveprotecting group with respect to the mercapto group. Therefore, thevicinity of the mercapto group of the sulfur-containing silane couplingagent is thought to be even bulkier than a conventional mercaptosilanedue to the siloxane structure of the skeleton, and also due to thepresence of B when the agent contains B.

The mercapto group of the sulfur-containing silane coupling agent isprotected by such a bulky structure, so the Mooney scorch time of therubber composition for a tire tread according to the present technologyis long, and the processing stability is secured.

However, in the present technology, it can be said that such a bulkystructure of the sulfur-containing silane coupling agent does notinhibit the acceleration of the vulcanization rate at the time ofvulcanization. It is thought that the mercapto group of thesulfur-containing silane coupling agent can interact and/or react withthe diene rubber as a result of heating or the like at the time ofvulcanization. Therefore, the composition of the present technology canachieve both processing stability and a fast vulcanization rate at thetime of vulcanization.

In addition, the sulfur-containing silane coupling agent may have betteraffinity and reactivity with silica than conventional mercaptosilanessince it has a hydrolyzable group represented by C and a siloxanestructure. Further, when the molecular weight of the sulfur-containingsilane coupling agent is within an appropriate range, it is anticipatedthat the affinity and reactivity with silica will be even better. It isthought that the rubber composition for a tire according to the presenttechnology achieves excellent wet performance and abrasion resistancedue to these factors.

In addition, it is thought that the affinity and reactivity with thesulfur-containing silane coupling agent are very high since a specificsilica is used in the present technology.

It is thought to be due to such reasons that the composition hasexcellent wet performance, abrasion resistance, and processability aswell as an excellent balance thereof.

The above mechanism is an inference by the inventors of the presentapplication, but if the mechanism is a mechanism other than thatdescribed above, it is still within the scope of the present technology.

The diene rubber contained in the composition of the present technologyis not particularly limited. Examples thereof include styrene butadienecopolymer rubber (SBR), natural rubber (NR), isoprene rubber (IR),butadiene rubber (BR), chloroprene rubber (CR), acrylonitrile butadienecopolymer rubber (NBR), ethylene-propylene-diene copolymer rubber(EPDM), styrene-isoprene copolymer rubber, isoprene-butadiene copolymerrubber, nitrile rubber, and hydrogenated nitrile rubber.

A single diene rubber can be used, or a combination of two or more typescan be used.

Of these, the diene rubber is preferably SBR and/or BR in that a tirewith superior wet performance and low rolling resistance can be producedand that the abrasion resistance is favorable.

The SBR is not particularly limited. The SBR may be modified by ahydroxy group, a polyorganosiloxane group, a carbonyl group, an aminogroup, or the like.

The weight average molecular weight of the SBR is preferably from500,000 to 2,000,000 and more preferably from 700,000 to 1,500,000 fromthe perspective of being able to achieve both grip performance andprocessability. In the present technology, the weight average molecularweight of the SBR is determined in terms of polystyrene by gelpermeation chromatography (GPC) using toluene as a solvent.

The SBR preferably contains from 30 to 50 mass % of an aromatic vinyl(called the “styrene quantity” hereafter) and has a vinyl bond contentof from 20 to 70 mass % in the conjugated diene in order to be able toproduce a tire having superior wet performance and low rollingresistance.

The content (proportion) of the SBR is preferably at least 50 mass % andmore preferably at least 60 mass % of the diene rubber in order to beable to produce a tire having superior wet performance and low rollingresistance.

The BR is not particularly limited. Examples thereof includeconventionally known substances.

The silica contained in the composition of the present technology has anitrogen adsorption specific surface area (N₂SA) of from 194 to 240 m²/gand a CTAB specific surface area (CTAB) of from 180 to 215 m²/g.

The N₂SA of the silica is preferably from 194 to 235 m²/g and morepreferably from 197 to 230 m²/g from the perspective of having superiorwet performance and excellent abrasion resistance.

When the N₂SA of the silica is less than 194 m²/g, the wet performanceis poor, which causes the reinforcement of the rubber composition to beinsufficient and causes the abrasion resistance and steering stabilityto be unsatisfactory. In addition, when the N₂SA of the silica exceeds240 m²/g, the dispersibility with respect to the diene rubber decreases,and the rolling resistance becomes large.

In the present technology, the N₂SA of the silica is determined inaccordance with JIS K6217-2.

The CTAB adsorption specific surface area (CTAB) of the silica ispreferably from 185 to 215 m²/g and more preferably from 185 to 210 m²/gfrom the perspective of having superior wet performance and excellentabrasion resistance.

When the CTAB of the silica is less than 180 m²/g, the wet performanceis poor, which causes the reinforcement of the rubber composition to beinsufficient and causes the abrasion resistance and steering stabilityto be unsatisfactory. In addition, when the CTAB of the silica exceeds215 m²/g, the dispersibility with respect to the diene rubber decreases,and the rolling resistance becomes large.

In the present technology, the CTAB of the silica was measured inaccordance with the CTAB adsorption method disclosed in JISK6217-3:2001.

The DBP (dibutyl phthalate) absorption number of the silica ispreferably at least 190 ml/100 g and more preferably from 200 to 250ml/100 g from the perspective of heaving superior wet performance, lowrolling resistance, and processability and excellent strength.

When the DBP absorption number of the silica is less than 190 ml/100 g,the processability and rubber strength are diminished.

In the present technology, the DBP absorption number of the silica isdetermined in accordance with the oil absorption number method Adescribed in JIS K6217-4.

The ratio (N₂SA/CTAB) of the N₂SA of the silica and the CTAB of thesilica is preferably from 0.9 to 1.4 and more preferably from 0.9 to 1.2from the perspective of having a superior wet performance, low rollingresistance, and processability and an excellent balance of reinforcementand dispersibility.

Examples of the silica contained in the composition of the presenttechnology include fumed silica, calcined silica, precipitated silica,ground silica, fused silica, colloidal silica, and surface-treatedsilica. A single silica can be used, or a combination of two or moretypes can be used.

In the present technology, the content of the silica is from 60 to 200parts by mass per 100 parts by mass of the diene rubber and ispreferably from 60 to 150 parts by mass, more preferably from 65 to 145parts by mass, and even more preferably from 70 to 140 parts by mass inthat the wet performance, low rolling resistance, and processability aresuperior and that the abrasion resistance and strength improve.

When the content of the silica is less than 60 parts by mass per 100parts by mass of the diene rubber, the wet performance is poor.

The sulfur-containing silane coupling agent contained in the compositionof the present technology will be described hereinafter. Thesulfur-containing silane coupling agent contained in the composition ofthe present technology is a polysiloxane represented by the followingformula (1).(A)_(a)(B)_(b)(C)_(c)(D)_(d)(R¹)_(e)SiO_((4-2a-b-c-d-e)/2)  (1)[Formula (1) is an average composition formula, wherein A is a divalentorganic group containing a sulfide group; B is a monovalent hydrocarbongroup having from 5 to 10 carbon atoms; C is a hydrolyzable group; D isan organic group containing a mercapto group; R¹ is a monovalenthydrocarbon group having from 1 to 4 carbon atoms; and a to e satisfythe relational expressions 0≦a<1, 0≦b<1, 0<c<3, 0<d<1, 0≦e<2, and0<2a+b+c+d+e<4. However, at least one of a and b is not 0.]

In the present technology, since the sulfur-containing silane couplingagent contains C, it has excellent affinity and/or reactivity withsilica.

Since the sulfur-containing silane coupling agent contains D, it caninteract and/or react with the diene rubber, which yields excellent wetperformance and abrasion resistance.

When the sulfur-containing silane coupling agent has A, the wetperformance and processability (in particular, the maintenance andprolongation of the Mooney scorch time) are superior, and the abrasionresistance is excellent.

When the sulfur-containing silane coupling agent contains B, themercapto group is protected, and the Mooney scorch time becomes longer,while at the same time, the processability is excellent due tooutstanding affinity with the rubber.

The sulfur-containing silane coupling agent contained in the compositionof the present technology has a siloxane skeleton as its skeleton.

In formula (1), A is a divalent organic group containing a sulfide group(also called a sulfide group-containing organic group hereafter). Theorganic group may be, for example, a hydrocarbon group optionally havinga hetero atom such as an oxygen atom, a nitrogen atom, or a sulfur atom.

Of these, a group represented by formula (2) is preferable.*—(CH₂)_(n)—S_(x)—(CH₂)_(n)—*  (2)

In formula (2), n is an integer from 1 to 10, among which an integerfrom 2 to 4 is preferable.

In formula (2), x is an integer from 1 to 6, among which an integer from2 to 4 is preferable.

In formula (2), * indicates a bond position.

Specific examples of the group represented by formula (2) include*—CH₂—S₂—CH₂—*, *—C₂H₄—S₂—C₂H₄—*, *—C₃H₆—S₂—C₃H₆—*, *—C₄H₈—S₂—C₄H₈—*,*—CH₂—S₄—CH₂—*, *—C₂H₄—S₄—C₂H₄—*, *—C₃H₆—S₄—C₃H₆—*, and*—C₄H₈—S₄—C₄H₈—*.

In formula (1), B is a monovalent hydrocarbon group having from 5 to 10carbon atoms, specific examples of which include hexyl groups, octylgroups, and decyl groups. Of these, B is preferably a monovalenthydrocarbon group having from 8 to 10 carbon atoms from the perspectiveof protecting the mercapto group, having a long Mooney scorch time,having superior processability, and having superior wet characteristicsand low rolling resistance.

In formula (1), C is a hydrolyzable group, specific examples of whichinclude alkoxy groups, phenoxy groups, carboxyl groups, and alkenyloxygroups. Of these, a group represented by the following formula (3) ispreferable.*—OR²  (3)

In formula (3), R² is an alkyl group having from 1 to 20 carbon atoms,an aryl group having from 6 to 10 carbon atoms, an aralkyl group (arylalkyl group) having from 6 to 10 carbon atoms, or an alkenyl grouphaving from 2 to 10 carbon atoms, among which an alkyl group having from1 to 5 carbon atoms is preferable. Specific examples of the alkyl grouphaving from 1 to 20 carbon atoms include a methyl group, an ethyl group,a propyl group, a butyl group, a hexyl group, an octyl group, a decylgroup, an octadecyl group, and the like. Specific examples of the arylgroup having from 6 to 10 carbon atoms include a phenyl group, a tolylgroup, and the like. Specific examples of the aralkyl group having from6 to 10 carbon atoms include a benzyl group, a phenylethyl group, andthe like. Specific examples of alkenyl groups having from 2 to 10 carbonatoms include vinyl groups, propenyl groups, and pentenyl groups.

In formula (3), * indicates a bond position.

In formula (1), D is an organic group containing a mercapto group. Ofthese, a group represented by the following formula (4) is preferable.*—(CH₂)_(m)—SH  (4)

In formula (4), m is an integer from 1 to 10, among which an integerfrom 1 to 5 is preferable.

In formula (4), * indicates a bond position.

Specific examples of the group represented by formula (4) include*—CH₂SH, *—C₂H₄SH, *—C₃H₆SH, *—C₄H₈SH, *—C₅H₁₀SH, *—C₆H₁₂SH, *—C₇H₁₄SH,*—C₈H₁₆SH, *—C₉H₁₈SH, and *—C₁₀H₂₀SH.

In formula (1), R¹ is a monovalent hydrocarbon group having from 1 to 4carbon atoms.

In formula (1), a to e satisfy the relational expressions 0≦a<1, 0≦b<1,0<c<3, 0<d<1, 0≦e<2, and 0<2a+b+c+d+e<4. However, at least one of a andb is not 0.

The value of (a) of the polysiloxane represented by the averagecomposition formula of formula (1) is preferably greater than 0 (0<a) inthat the resulting processability is superior. That is, a case in whichthe substance has a sulfide-containing organic group is a preferredaspect. Of these, it is preferable for the expression 0<a≦0.50 to besatisfied in that the processability is even better and the wetperformance and low rolling resistance are also superior.

In addition, the value of (a) of the polysiloxane represented by theaverage composition formula of formula (1) is preferably 0 in that thewet performance and low rolling resistance are superior. That is, a casein which the substance does not have a sulfide-containing organic groupis a preferred aspect.

In formula (1), b is preferably greater than 0 and preferably satisfiesthe expression 0.10≦b≦0.89 in that the wet characteristics, low rollingresistance, and processability are superior.

In formula (1), c preferably satisfies the expression 1.2≦c≦2.0 in thatthe wet characteristics, low rolling resistance, and processability aresuperior and the silica dispersibility is superior.

In formula (1), d preferably satisfies the expression 0.1≦d≦0.8 in thatthe wet characteristics, low rolling resistance, and processability aresuperior.

The polysiloxane represented by the average composition formula offormula (1) is preferably a polysiloxane in which, in formula (1), A isa group represented by formula (2), C is a group represented by formula(3), and D is a group represented by formula (4), and B is morepreferably a monovalent hydrocarbon group having from 8 to 10 carbonatoms in that the silica dispersibility is good and the processabilityis superior.

The weight average molecular weight of the polysiloxane represented bythe average composition formula of formula (1) is preferably from 500 to2,300 and more preferably from 600 to 1,500 from the perspective ofhaving superior wet performance, low rolling resistance, andprocessability. The molecular weight of the polysiloxane is the weightaverage molecular weight determined in terms of polystyrene by gelpermeation chromatography (GPC) using toluene as a solvent.

The mercapto equivalent weight of the polysiloxane represented by theaverage composition formula of formula (1) determined by the aceticacid/potassium iodide/potassium iodate addition-sodium thiosulfatesolution titration method is preferably from 550 to 1900 g/mol and morepreferably from 600 to 1800 g/mol, from the perspective of havingexcellent vulcanization reactivity.

The method for producing the polysiloxane is not particularly limited.For example, it may be produced by hydrolyzing and condensing anorganosilicon compound containing at least a silane coupling agenthaving a mercapto group as a starting material.

A specific example is a method of hydrolyzing and condensing anorganosilicon compound represented by the following formula (6) (forexample, p=5 to 10) and an organosilicon compound represented by thefollowing formula (7). Further, an organosilicon compound represented bythe following formula (5) may also be used. In addition, anorganosilicon compound represented by formula (6) (for example, p=1 to4) may also be used.

Of these, it is preferable to use at least an organosilicon compoundrepresented by formula (6) (for example, p=5 to 10) and organosiliconcompounds represented by formula (7) and formula (5) in that the scorchresistance is superior.

In addition, it is preferable to use at least an organosilicon compoundrepresented by formula (6) (for example, p=5 to 10) and an organosiliconcompound represented by formula (7) in that the wet performance issuperior.

In formula (5), R⁵¹ is an alkyl group having from 1 to 20 carbon atoms,an aryl group having from 6 to 10 carbon atoms, or an alkenyl grouphaving from 2 to 10 carbon atoms, among which an alkyl group having from1 to 5 carbon atoms is preferable. Specific examples of the alkyl grouphaving from 1 to 20 carbons include a methyl group, an ethyl group, apropyl group, a butyl group, a hexyl group, an octyl group, a decylgroup, an octadecyl group, and the like. Specific examples of arylgroups having from 6 to 10 carbon atoms include phenyl groups, tolylgroups, and naphthyl groups. Specific examples of alkenyl groups havingfrom 2 to 10 carbon atoms include vinyl groups, propenyl groups, andpentenyl groups.

In formula (5), R⁵² is an alkyl group having from 1 to 10 carbon atomsor an aryl group having from 6 to 10 carbon atoms. Specific examples ofalkyl groups having from 1 to 10 carbon atoms include methyl groups,ethyl groups, propyl groups, butyl groups, hexyl groups, octyl groups,and decyl groups. Specific examples of aryl groups having from 6 to 10carbon atoms are the same as those of R⁵¹ described above.

In formula (5), the definition and preferred aspects of n are the sameas those of n in formula (2) described above.

In formula (5), the definition and preferred aspects of x are the sameas those of x in formula (2) described above.

In formula (5), y is an integer from 1 to 3.

Specific examples of the organosilicon compound represented by formula(5) include bis(trimethoxysilylpropyl)tetrasulfide,bis(triethoxysilylpropyl)tetrasulfide,bis(trimethoxysilylpropyl)disulfide, andbis(triethoxysilylpropyl)disulfide.

In formula (6), the definition, specific examples, and preferred aspectsof R⁶¹ are the same as those of R⁵¹ described above.

In formula (6), the definition, specific examples, and preferred aspectsof R⁶² are the same as those of R⁵² described above.

In formula (6), the definition of z is the same as that of y describedabove.

In formula (6), p is an integer from 1 to 10. Here, p is preferably aninteger from 5 to 10 from the perspective of having superior wetperformance, low rolling resistance, and processability and havingexcellent affinity with the diene rubber.

As the organosilicon compound represented by formula (6), anorganosilicon compound in which p is an integer from 1 to 4 and/or anorganosilicon compound in which p is an integer from 5 to 10 can beused.

Specific examples of the organosilicon compound represented by formula(6) (p is an integer from 5 to 10) include pentyltrimethoxysilane,pentylmethyldimethoxysilane, pentyltriethoxysilane,pentylmethyldiethoxysilane, hexyltrimethoxysilane,hexylmethyldimethoxysilane, hexyltriethoxysilane,hexylmethyldiethoxysilane, octyltrimethoxysilane,octylmethyldimethoxysilane, octyltriethoxysilane,octylmethyldiethoxysilane, decyltrimethoxysilane,decylmethyldimethoxysilane, decyltriethoxysilane, anddecylmethyldiethoxysilane.

Specific examples of the organosilicon compound represented by formula(6) (p is an integer from 1 to 4) include methyltrimethoxysilane,dimethyldimethoxysilane, methyltriethoxysilane,methylethyldiethoxysilane, propyltrimethoxysilane,propylmethyldimethoxysilane, and propylmethyldiethoxysilane.

In formula (7), the definition, specific examples, and preferred aspectsof R⁷¹ are the same as those of R⁵¹ described above.

In formula (7), the definition, specific examples, and preferred aspectsof R⁷² are the same as those of R⁵² described above.

In formula (7), the definition and preferred aspects of m are the sameas those of m in formula (4) described above.

In formula (7), the definition of w is the same as that of y describedabove.

Specific examples of the organosilicon compound represented by formula(7) include α-mercaptomethyltrimethoxysilane,α-mercaptomethylmethyldimethoxysilane, α-mercaptomethyltriethoxysilane,α-mercaptomethylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane,γ-mercaptopropylmethyldimethoxysilane, γ-mercaptopropyltriethoxysilane,and γ-mercaptopropylmethyldiethoxysilane.

When a silane coupling agent having a mercapto group [for example, anorganosilicon compound represented by formula (7)] and a silane couplingagent other than a silane coupling agent having a sulfide group or amercapto group [for example, an organosilicon compound represented byformula (6)], for example, are used in combination as the organosiliconcompounds used to produce the polysiloxane, the mixing ratio (molarratio) of the silane coupling agent having a mercapto group and thesilane coupling agent other than a silane coupling agent having asulfide group or a mercapto group (silane coupling agent having amercapto group/silane coupling agent other than a silane coupling agenthaving a sulfide group or a mercapto group) is preferably from 1.1/8.9to 6.7/3.3 and more preferably from 1.4/8.6 to 5.0/5.0 from theperspective of having superior wet performance, low rolling resistance,and processability.

When a silane coupling agent having a mercapto group [for example, anorganosilicon compound represented by formula (7)] and a silane couplingagent having a sulfide group [for example, an organosilicon compoundrepresented by formula (5)], for example, are used in combination as theorganosilicon compounds used to produce the polysiloxane, the mixingratio (molar ratio) of the silane coupling agent having a mercapto groupand the silane coupling agent having a sulfide group (silane couplingagent having a mercapto group/silane coupling agent having a sulfidegroup) is preferably from 2.0/8.0 to 8.9/1.1 and more preferably from2.5/7.5 to 8.0/2.0 from the perspective of having superior wetperformance, low rolling resistance, and processability.

When a silane coupling agent having a mercapto group [for example, anorganosilicon compound represented by formula (7)], a silane couplingagent having a sulfide group [for example, an organosilicon compoundrepresented by formula (5)], and a silane coupling agent other than asilane coupling agent having a sulfide group or a mercapto group [forexample, an organosilicon compound represented by formula (6)], forexample, are used in combination as the organosilicon compounds used toproduce the polysiloxane, the amount of the silane coupling agent havinga mercapto group is preferably from 10.0 to 73.0% of the total amount(moles) of the three silane coupling agents listed above. The amount ofthe silane coupling agent having a sulfide group is preferably from 5.0to 67.0% of the total amount of the three agents listed above. Theamount of the silane coupling agent other than a silane coupling agenthaving a sulfide group or a mercapto group is preferably from 16.0 to85.0% of the total amount of the three agents listed above.

A solvent may be used as necessary when producing the polysiloxanedescribed above. The solvent is not particularly limited, but specificexamples include aliphatic hydrocarbon solvents such as pentane, hexane,heptane, and decane, ether solvents such as diethyl ether,tetrahydrofuran, and 1,4-dioxane, amide solvents such as formamide,dimethylformamide, and N-methylpyrrolidone, aromatic hydrocarbonsolvents such as benzene, toluene, and xylene, and alcohol solvents suchas methanol, ethanol, and propanol.

In addition, a catalyst may be used as necessary when producing thepolysiloxane represented by formula (1).

In the present technology, examples of catalysts that can be used whenforming the polysiloxane represented by formula (1) include acidiccatalysts such as hydrochloric acid and acetic acid; Lewis acidcatalysts such as ammonium fluoride; alkali metal salts such as sodiumhydroxide, potassium hydroxide, sodium carbonate, sodium acetate,potassium acetate, sodium hydrogen carbonate, potassium carbonate,potassium hydrogen carbonate, calcium carbonate, sodium methoxide, andsodium ethoxide; alkali earth metal salts; and amine compounds such astriethylamine, tributylamine, pyridine, and 4-dimethylaminopyridine.

The catalyst described above is preferably not an organic metal compoundcontaining Sn, Ti, or Al as a metal. When such an organic metal compoundis used, the metal is introduced into the polysiloxane skeleton, and itmay not be possible to obtain the specific polysiloxane described above(in which no metals other than silicon atoms (for example, Sn, Ti, orAl) are present in the skeleton).

When an organic metal compound containing Sn, Ti, or Al is not used as acatalyst, metals derived from the catalyst are not introduced into themolecules of the polysiloxane (for example, metals are not introducedinto the polysiloxane skeleton), and the rubber composition for a tiretread according to the present technology is not hardened or gelified bymoisture in the air in either a normal atmosphere or a high-humidityenvironment, which yields excellent storage stability.

The amount of the catalyst is preferably from 0.01 to 10 parts by massand more preferably from 0.05 to 1 part by mass per 100 parts by mass ofthe organosilicon compound used as a starting material from theperspective of having superior wet performance, low rolling resistance,and processability and having excellent storage stability.

The sulfur-containing silane coupling agent can be used alone or as acombination of two or more types.

In the rubber composition for a tire tread according to the presenttechnology, the content of the sulfur-containing silane coupling agentis from 1 to 20 mass % of the content of the silica and is preferablyfrom 2 to 20 mass %, more preferably from 3 to 18 mass %, even morepreferably from 4 to 16 mass %, and particularly preferably from 5 to 14mass % from the perspective of having superior wet performance, lowrolling resistance, and processability.

When the content of the sulfur-containing silane coupling agent exceeds20 mass % of the silica content, the processability is poor.

The composition of the present technology preferably further contains aterpene resin in that that balance of the wet performance, low rollingresistance, and processability is superior.

The terpene resin is preferably an aromatic modified terpene resin. Theterpene resin and aromatic modified terpene resin are not particularlylimited. Examples thereof include conventionally known substances.

The softening point of the terpene resin (in particular, an aromaticmodified terpene resin) is preferably from 60 to 150° C. and morepreferably from 70 to 130° C. from the perspective of having superiorwet performance, low rolling resistance, and processability.

The terpene resin may be used alone or as a combination of two or moretypes.

The amount of the terpene resin is preferably from 1 to 30 parts by massand more preferably from 3 to 20 parts by mass per 100 parts by mass ofthe diene rubber component from the perspective of having superior wetperformance, low rolling resistance, and processability.

The composition of the present technology may further contain additivesas necessary within a scope that does not inhibit the effect or purposethereof. Examples of additives include various additives typically usedin rubber compositions for tire treads such as silane coupling agentsother than the polysiloxane represented by the average compositionformula of formula (1) and contained rubber compositions for tire treadsof the present technology, fillers other than silica (for example,carbon black, clay, mica, talc, calcium carbonate, aluminum hydroxide,aluminum oxide, and titanium oxide), zinc oxide, stearic acid, antiagingagents, processing aids, aroma oils, process oils, liquid polymers,thermosetting resins, vulcanizing agents, and vulcanizationaccelerators.

When the composition of the present technology further contains a carbonblack, the carbon black is not particularly limited. Examples thereofinclude conventionally known substances. A single carbon black can beused or a combination of two or more carbon blacks can be used.

The method for producing the composition of the present technology isnot particularly limited, and specific examples thereof include a methodwhereby each of the above-mentioned components is kneaded using apublicly known method and device (e.g. Banbury mixer, kneader, roll, andthe like).

In addition, the composition of the present technology can be vulcanizedor crosslinked under conventional, publicly known vulcanizing orcrosslinking conditions.

Next, the pneumatic tire of the present technology will be described.

The pneumatic tire of the present technology is a pneumatic tire havingtire treads formed by using the rubber composition for a tire treadaccording to the present technology.

The pneumatic tire of the present technology will be describedhereinafter with reference to the attached drawings. The pneumatic tireof the present technology is not limited to the attached drawings.

FIG. 1 is a partial cross-sectional schematic view of a tire thatrepresents one embodiment of the pneumatic tire of the presenttechnology.

In FIG. 1, reference number 1 denotes a bead portion, reference number 2denotes a side wall portion, and reference number 3 denotes a tiretread.

In addition, a carcass layer 4, in which a fiber cord is embedded, ismounted between a left-right pair of bead portions 1, and ends of thecarcass layer 4 are wound by being folded around bead cores 5 and a beadfiller 6 from an inner side to an outer side of the tire.

In the tire tread portion 3, a belt layer 7 is provided along the entireperiphery of the tire on the outer side of the carcass layer 4.

Additionally, rim cushions 8 are provided in parts of the bead portions1 that are in contact with a rim.

The pneumatic tire of the present technology is not particularly limitedwith the exception that the rubber composition for a tire treadaccording to the present technology is used for the tire treads of apneumatic tire, and, for example, the tire can be produced with aconventionally known method. In addition to ordinary air or air with anadjusted oxygen partial pressure, inert gasses such as nitrogen, argon,and helium can be used as the gas with which the tire is filled.

EXAMPLES

The present technology will be described in further detail hereinafterusing working examples. The present technology is not limited to theseworking examples.

<Production Method for Polysiloxane 1>

107.8 g (0.2 mol) of bis(triethoxysilylpropyl)tetrasulfide (KBE-846,manufactured by Shin-Etsu Chemical Co., Ltd.), 190.8 g (0.8 mol) ofγ-mercaptopropyl triethoxysilane (KBE-803, manufactured by Shin-EtsuChemical Co., Ltd.), 442.4 g (1.6 mol) of octyl triethoxysilane(KBE-3083, manufactured by Shin-Etsu Chemical Co., Ltd.), and 190.0 g ofethanol were placed in a 2 L separable flask provided with an agitator,a reflux condenser, a dropping funnel and a thermometer, and then amixed solution containing 37.8 g (2.1 mol) of 0.5 N hydrochloric acidand 75.6 g of ethanol was added in a dropwise manner at roomtemperature. It was then stirred for 2 hours at 80° C. Then, it wasfiltered, and 17.0 g of 5% KOH/EtOH solution was added in a dropwisemanner, and stirred for 2 hours at 80° C. Then, by vacuum concentrationand filtration, 480.1 g of polysiloxane in the form of a browntransparent liquid was obtained. As a result of performing measurementsby GCP, the average molecular weight of the obtained polysiloxane was840, and the average degree of polymerization was 4.0 (preset degree ofpolymerization: 4.0). In addition, as a result of measuring the mercaptoequivalent weight of the obtained polysiloxane by an aceticacid/potassium iodide/potassium iodate addition/sodium thiosulfatesolution titration method, the mercapto equivalent weight was 730 g/mol,and it was thus confirmed that the preset mercapto group content wasachieved. The polysiloxane obtained as described above is represented bythe following average composition formula.(—C₃H₆—S₄—C₃H₆—)_(0.071)(—C₈H₁₇)_(0.571)(—OC₂H₅)_(1.50)(—C₃H₆SH)_(0.286)SiO_(0.75)

The obtained polysiloxane was used as polysiloxane 1.

<Production Method for Polysiloxane 2>

190.8 g (0.8 mol) of γ-mercaptopropyl triethoxysilane (KBE-803,manufactured by Shin-Etsu Chemical Co., Ltd.), 442.4 g (1.6 mol) ofoctyl triethoxysilane (KBE-3083, manufactured by Shin-Etsu Chemical Co.,Ltd.), and 162.0 g of ethanol were placed in a 2 L separable flaskprovided with an agitator, a reflux condenser, a dropping funnel and athermometer, and then a mixed solution containing 32.4 g (1.8 mol) of0.5 N hydrochloric acid and 75.6 g of ethanol was added in a dropwisemanner at room temperature. It was then stirred for 2 hours at 80° C.Then, it was filtered, and 14.6 g of 5% KOH/EtOH solution was added in adropwise manner, and stirred for 2 hours at 80° C. Then, by vacuumconcentration and filtration, 412.3 g of polysiloxane in the form of acolorless transparent liquid was obtained. As a result of performingmeasurements by GPC, the average molecular weight of the obtainedpolysiloxane was 850, and the average degree of polymerization was 4.0(preset degree of polymerization: 4.0). In addition, the mercaptoequivalent weight of the polysiloxane measured by an aceticacid/potassium iodide/potassium iodate addition/sodium thiosulfatesolution titration method was 650 g/mol, and it was thus confirmed thatthe preset mercapto group content was achieved. The polysiloxaneobtained as described above is represented by the following averagecomposition formula.(—C₈H₁₇)_(0.667)(—OC₂H₅)_(1.50)(—C₃H₆SH)_(0.333)SiO_(0.75)

The obtained polysiloxane was used as polysiloxane 2.

<Production Method for Polysiloxane 3>

107.8 g (0.2 mol) of bis(triethoxysilylpropyl)tetrasulfide (KBE-846,manufactured by Shin-Etsu Chemical Co., Ltd.), 190.8 g (0.8 mol) ofγ-mercaptopropyl triethoxysilane (KBE-803, manufactured by Shin-EtsuChemical Co., Ltd.), 442.4 g (1.6 mol) of octyl triethoxysilane(KBE-3083, manufactured by Shin-Etsu Chemical Co., Ltd.), and 190.0 g ofethanol were placed in a 2 separable flask provided with an agitator, areflux condenser, a dropping funnel and a thermometer, and then a mixedsolution containing 42.0 g (2.33 mol) of 0.5 N hydrochloric acid and75.6 g of ethanol was added in a dropwise manner at room temperature. Itwas then stirred for 2 hours at 80° C. Then, it was filtered, and 18.9 gof 5% KOH/EtOH solution was added in a dropwise manner, and stirred for2 hours at 80° C. Then, by vacuum concentration and filtration, 560.9 gof polysiloxane in the form of a brown transparent liquid was obtained.As a result of performing measurements by GPC, the average molecularweight of the obtained polysiloxane was 1,220, and the average degree ofpolymerization was 6.0 (preset degree of polymerization: 6.0). Inaddition, the mercapto equivalent weight of the polysiloxane measured byan acetic acid/potassium iodide/potassium iodate addition/sodiumthiosulfate solution titration method was 710 g/mol, and it was thusconfirmed that the preset mercapto group content was achieved. Thepolysiloxane obtained as described above is represented by the followingaverage composition formula.(—C₃H₆—S₄—C₃H₆—)_(0.071)(—C₈H₁₇)_(0.571)(—OC₂H₅)_(1.334)(—C₃H₆SH)_(0.286)SiO_(0.833)

The obtained polysiloxane was used as polysiloxane 3.

(Comparative Polysiloxane 1)

A polysiloxane was obtained by hydrolyzing3-mercaptopropyltrimethoxysilane (0.1 mol) with water and a concentratedhydrochloric acid aqueous solution and then adding and condensingethoxymethylpolysiloxane (100 g). The obtained polysiloxane was used ascomparative polysiloxane 1.

The comparative polysiloxane 1 has a structure in which the methoxygroups of 3-mercaptopropyltrimethoxysilane and the ethoxy groups ofethoxymethylpolysiloxane are condensed. That is, the only monovalenthydrocarbon group of the comparative polysiloxane 1 is a methyl group.In addition, the comparative polysiloxane 1 does not have a divalentorganic group containing a sulfide group.

(Comparative Polysiloxane 2)

A polysiloxane was obtained by hydrolyzingbis(3-(triethoxysilyl)propyl)tetrasulfide (0.1 mol) with water and aconcentrated hydrochloric acid aqueous solution and then adding andcondensing ethoxymethylpolysiloxane (100 g). The obtained polysiloxanewas used as comparative polysiloxane 2.

The comparative polysiloxane 2 has a structure in which the ethoxygroups of bis(3-(triethoxysilyl)propyl)tetrasulfide and the ethoxygroups of ethoxymethylpolysiloxane are condensed. That is, the onlymonovalent hydrocarbon group of the comparative polysiloxane 2 is amethyl group. In addition, the comparative polysiloxane 2 does not havea divalent organic group containing a mercapto group.

<Production of the Rubber Composition for Tire Treads>

The components shown in the following table were blended at theproportions (parts by mass) shown in the table.

Specifically, a master batch was obtained by first mixing the componentsshown in the following table, excluding the sulfur and the vulcanizationaccelerator, for 10 minutes in a 1.7-liter closed-type Banbury mixer,discharging the mixture, and then cooling the mixture to roomtemperature. Further, sulfur and a vulcanization accelerator were mixedinto the resulting master batch using the Banbury mixer described aboveso as to obtain a rubber composition for a tire tread.

In the table, the numerical values in parentheses in the rows of thecomparative silane coupling agents 1 and 2 and the silane couplingagents 1 to 3 indicate the mass % of each component with respect to theamount of silica.

The following evaluations were performed using the rubber compositionfor a tire tread produced as described above. The results are shown inthe tables below.

<tan δ (0° C.)> (Indicator of Wet Performance)

A vulcanized rubber sheet was produced by press-vulcanizing the rubbercomposition for a tire tread (unvulcanized) produced as described abovefor 20 minutes at 160° C. in a metal mold (15 cm×15 cm×0.2 cm).

The value of tan δ (0° C.) was measured for the produced vulcanizedrubber sheet under conditions with an elongation deformation distortionof 10%±2%, an oscillation frequency of 20 Hz, and a temperature of 0° C.using a viscoelastic spectrometer (produced by Iwamoto Manufacturing) inaccordance with JIS K6394:2007.

The results are displayed as an index when the value of tan δ (0° C.) ofa reference example is defined as 100. Larger indexes indicate largertan δ (0° C.) values, which in turn indicates excellent wet performancewhen used in a tire.

<tan δ (60° C.)> (Indicator of Low Rolling Resistance)

A vulcanized rubber sheet was produced by press-vulcanizing the rubbercomposition for a tire tread (unvulcanized) produced as described abovefor 20 minutes at 160° C. in a metal mold (15 cm×15 cm×0.2 cm).

The value of tan δ (60° C.) was measured for the produced vulcanizedrubber sheet under conditions with an elongation deformation distortionof 10%±2%, an oscillation frequency of 20 Hz, and a temperature of 60°C. using a viscoelastic spectrometer (produced by Iwamoto Manufacturing)in accordance with JIS K6394:2007.

The results are displayed as an index when the value of tan δ (60° C.)of a reference example is defined as 100. Smaller index values indicatesmaller tan δ (60° C.) values, which in turn indicates excellent lowrolling resistance when used in a pneumatic tire.

<Mooney Viscosity>

The Mooney viscosity of the rubber composition (unvulcanized) for a tiretread produced as described above was measured under conditions with apreheating time of 1 minute, a rotor rotation time of 4 minutes, and atest temperature of 100° C. using an L-shaped rotor in accordance withJIS K6300-1:2001.

The results are displayed as an index when the value of a referenceexample is defined as 100.

<Mooney Scorch> (Indicator of Scorch Resistance)

The scorch time of the rubber composition (unvulcanized) for a tiretread produced as described above was measured under conditions with atest temperature of 125° C. using an L-shaped rotor in accordance withJIS K6300-1:2001.

The results are displayed as an index when the scorch time of areference example is defined as 100. Larger indexes indicate longerscorch times, which in turn indicates excellent scorch resistance(processability).

TABLE 1 Reference Comparative Comparative Comparative Example Example 1Example 2 Example 3 SBR (E581) 96.3(70)   96.3(70)   96.3(70)  96.3(70)   (numbers in parentheses to the right indicate the rubbercontent) BR 30 30 30 30 Comparative silica 1 80 80 Comparative silica 2Silica 1 80 Silica 2 80 Carbon black 10 10 10 10 Comparative silane8.8(11%) 8.8(11%) 8.8(11%) coupling agent 1 Comparative silane 8.8(11%)coupling agent 2 Silane coupling agent 1 Silane coupling agent 2 Silanecoupling agent 3 Comparative silane coupling agent 3 Comparative silanecoupling agent 4 Stearic acid 2.5 2.5 2.5 2.5 Zinc oxide 2.5 2.5 2.5 2.5Antiaging agent 2 2 2 2 Terpene resin Process oil 10 10 10 10Vulcanization 2 2 2 2 accelerator 1 Vulcanization 1 1 1 1 accelerator 2Sulfur 2 2 2 2 Evaluated Item Wet performance 100 93 101 97 Rollingresistance 100 97 98 94 Mooney viscosity 100 92 97 108 Mooney scorch 100101 99 87 Comparative Comparative Comparative Comparative Example 4Example 5 Example 6 Example 7 SBR (E581) 96.3(70)   96.3(70)  96.3(70)   96.3(70)    (numbers in parentheses to the right indicate therubber content) BR 30 30 30 30 Comparative silica 1 80 Comparativesilica 2 80 Silica 1 80 80 Silica 2 Carbon black 10 10 10 10 Comparativesilane coupling agent 1 Comparative silane 8.8(11%) coupling agent 2Silane coupling agent 1 8.8(11%) 8.8(11%) Silane coupling agent 220(25%) Silane coupling agent 3 Comparative silane coupling agent 3Comparative silane coupling agent 4 Stearic acid 2.5 2.5 2.5 2.5 Zincoxide 2.5 2.5 2.5 2.5 Antiaging agent 2 2 2 2 Terpene resin Process oil10 10 10 10 Vulcanization 2 2 2 2 accelerator 1 Vulcanization 1 1 1 1accelerator 2 Sulfur 2 2 2 2 Evaluated Item Wet performance 104 95 98114 Rolling resistance 96 94 100 95 Mooney viscosity 113 78 80 101Mooney scorch 89 110 117 98 Comparative Comparative Comparative WorkingExample 8 Example 9 Example 10 Example 1 SBR (E581) 96.3(70)  96.3(70)   96.3(70)   96.3(70)   (numbers in parentheses to the rightindicate the rubber content) BR 30 30 30 30 Comparative silica 1Comparative silica 2 Silica 1 80 80 80 Silica 2 40 Carbon black 50 10 1010 Comparative silane coupling agent 1 Comparative silane coupling agent2 Silane coupling agent 1 8.8(11%) Silane coupling agent 2 4.4(11%)Silane coupling agent 3 Comparative silane 8.8(11%) coupling agent 3Comparative silane 8.8(11%) coupling agent 4 Stearic acid 2.5 2.5 2.52.5 Zinc oxide 2.5 2.5 2.5 2.5 Antiaging agent 2 2 2 2 Terpene resinProcess oil 10 10 10 10 Vulcanization 2 2 2 2 accelerator 1Vulcanization 1 1 1 1 accelerator 2 Sulfur 2 2 2 2 Evaluated Item Wetperformance 83 99 98 106 Rolling resistance 105 98 100 94 Mooneyviscosity 81 98 99 80 Mooney scorch 116 102 101 115 Working WorkingWorking Working Working Example 2 Example 3 Example 4 Example 5 Example6 SBR (E581) 96.3(70)   96.3(70)   96.3(70)   96.3(70)  96.3(70)  (numbers in parentheses to the right indicate the rubber content) BR 3030 30 30 30 Comparative silica 1 Comparative silica 2 Silica 1 80 80 80Silica 2 80 80 Carbon black 10 10 10 10 10 Comparative silane couplingagent 1 Comparative silane coupling agent 2 Silane coupling agent 18.8(11%) 12.8(16%) Silane coupling agent 2 8.8(11%) Silane couplingagent 3 8.8(11%) 8.8(11%) Comparative silane coupling agent 3Comparative silane coupling agent 4 Stearic acid 2.5 2.5 2.5 2.5 2.5Zinc oxide 2.5 2.5 2.5 2.5 2.5 Antiaging agent 2 2 2 2 2 Terpene resin 5Process oil 10 10 10 10 10 Vulcanization 2 2 2 2 2 accelerator 1Vulcanization 1 1 1 1 1 accelerator 2 Sulfur 2 2 2 2 2 Evaluated ItemWet performance 107 105 108 113 112 Rolling resistance 92 96 91 88 98Mooney viscosity 84 76 77 87 74 Mooney scorch 112 118 115 108 113

The details of each of the components shown in Table 1 are as follows.

-   -   SBR: Styrene-butadiene rubber, E581 (oil extending quantity per        100 parts by mass of the rubber component: 37.5 parts by mass        (rubber content per 96.3 parts by mass: 70 parts by mass),        weight average molecular weight: 1,200,000, styrene content: 37        mass %, vinyl bond content: 43%, manufactured by the Asahi Kasei        Corporation)    -   BR: Butadiene rubber: Nippol BR 1220 (manufactured by the Zeon        Corporation)    -   Comparative silica 1: Zeosil 1165 MP manufactured by the Rhodia        Corporation; silica having a nitrogen adsorption specific        surface area of 160 m²/g, a CTAB specific surface area of 159        m²/g, a DBP absorption number of 200 ml/100 g, and N₂SA/CTAB of        1.0    -   Comparative silica 2: Nipsil AQ manufactured by the Toso Silica        Corporation; silica having a nitrogen adsorption specific        surface area of 211 m²/g, a CTAB specific surface area of 160        m³/g, a DBP absorption number of 193 ml/100 g, and N₂SA/CTAB of        1.3    -   Silica 1: Premium 200MP manufactured by the Rhodia Corporation;        silica having a nitrogen adsorption specific surface area of 207        m²/g, a CTAB specific surface area of 198 m²/g, a DBP absorption        number of 206 ml/100 g, and N₂SA/CTAB of 1.0    -   Silica 2: Ultrasil 9000GR manufactured by Evonik Degussa, silica        having a nitrogen adsorption specific surface area of 213 m²/g,        a CTAB specific surface area of 193 m²/g, a DBP absorption        number of 220 ml/100 g, and N₂SA/CTAB of 1.1    -   Carbon black: Show Black N339 (CTAB adsorption specific surface        area=90 m²/g, manufactured by Cabot Japan)    -   Comparative silane coupling agent 1: Si363 (manufactured by        Evonik Degussa)    -   Comparative silane coupling agent 2:        3-mercaptopropyltrimethoxysilane (KBM-803, manufactured by        Shin-Etsu Chemical Co., Ltd.)    -   Silane coupling agent 1: polysiloxane 1 produced as described        above    -   Silane coupling agent 2: polysiloxane 2 produced as described        above    -   Silane coupling agent 3: polysiloxane 3 produced as described        above    -   Comparative silane coupling agent 3: comparative polysiloxane 1        synthesized as described above    -   Comparative silane coupling agent 4: comparative polysiloxane 4        synthesized as described above    -   Stearic acid: stearic acid beads (manufactured by Nippon Oil &        Fats Co., Ltd.)    -   Zinc oxide: Type 3 zinc flower (manufactured by Seido Chemical        Industry Ltd.)    -   Antiaging agent:        N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine (Santoflex        6PPD, manufactured by Flexsys)    -   Terpene resin: YS Resin TO125 (manufactured by Yasuhara Chemical        Co., Ltd.) (aromatic modified terpene resin, softening point:        125° C.)    -   Process oil: Extract No. 4 S (manufactured by Showa Shell Seikyu        K.K.)    -   Vulcanization accelerator 1: N-cyclohexyl-2-benzothiazolyl        sulfenamide (NOCCELER CZ-G, manufactured by Ouchi Shinko        Chemical Industrial Co., Ltd.)    -   Vulcanization accelerator 2: 1,3-diphenylguanidine (Soxinol D-G,        manufactured by Sumitomo Chemical Co., Ltd.)    -   Sulfur: “Golden Flower” oil-treated sulfur powder (manufactured        by Tsurumi Chemical Industry Co., Ltd.)

As is clear from the results shown in Table 1, Comparative Example 3,which contains the comparative silane coupling agent 2 other than apolysiloxane represented by the average composition formula of formula(1) as a silane coupling agent containing sulfur and contains thecomparative silica 1 having an N₂SA of less than 194 m²/g and a CTAB ofless than 180 m²/g, demonstrated wet performance and processabilityinferior to those of the reference example. Comparative Example 4, whichcontains the comparative silane coupling agent 2, demonstrated wetperformance slightly superior to that of Comparative Example 3, butdemonstrated processability inferior to that of the reference example.

Comparative Example 1, which contains the comparative silane couplingagent 1 other than a polysiloxane represented by the average compositionformula of formula (1) as a silane coupling agent containing sulfur andcontains the comparative silica 1, demonstrated wet performance inferiorto that of the reference example. Comparative Example 2, which containsthe comparative silane coupling agent 1, demonstrated wet performancesuperior to that of Comparative Example 1, but demonstrated scorchresistance inferior to that of the reference example.

Comparative Examples 5 and 6, which contain the comparative silica 1 andthe comparative silica 2, demonstrated wet performance inferior to thatof the reference example.

Comparative Example 7, in which the content of the sulfur-containingsilane coupling agent exceeds 20 mass % of the silica content,demonstrated processability inferior to that of the reference example.

Comparative Example 8, in which the silica content is less than 60 partsby mass per 100 parts by mass of the diene rubber, demonstrated wetperformance and low rolling resistance inferior to those of thereference example.

Comparative Examples 9 and 10, which contain the comparative silanecoupling agent 3 or 4, demonstrated wet performance inferior to that ofthe reference example.

In contrast, the wet performance, low rolling resistance, andprocessability were all excellent in Working Examples 1 to 6.

As described above, the reference example and Comparative Examples 2 and4, in which a conventional sulfur-containing silane coupling agent and asilica with a small particle size are used in combination, can yieldproducts with excellent wet performance, low rolling resistance, andprocessability to a certain degree. However, when a polysiloxanerepresented by the average composition formula of formula (1) and asilica having an N₂SA and CTAB within prescribed ranges are used incombination as a sulfur-containing silane coupling agent, the effects onwet performance, low rolling resistance, and processability aredemonstrated to an even greater degree than when a conventionalsulfur-containing silane coupling agent and silica are used incombination. Therefore, it is thought that a combination of apolysiloxane represented by the average composition formula of formula(1) and a silica having an N₂SA and CTAB within prescribed ranges as asulfur-containing silane coupling agent very substantially contributesto the excellent effects on wet performance, low rolling resistance, andprocessability and that the tire performance and processability can bebalanced to a high degree.

In addition, Working Example 6, which further contains an aromaticmodified terpene resin, demonstrates wet performance even superior tothat of Working Example 3 and can suppress increases in viscosity.

What is claimed is:
 1. A rubber composition for a tire tread comprisinga diene rubber, silica, and a sulfur-containing silane coupling agent; anitrogen adsorption specific surface area of the silica being from 194to 240 m²/g; a CTAB specific surface area of the silica being from 180to 215 m²/g; a content of the silica being from 60 to 200 parts by massper 100 parts by mass of the diene rubber; the sulfur-containing silanecoupling agent being a polysiloxane represented by the following formula(1); and a content of the sulfur-containing silane coupling agent beingfrom 1 to 20 mass % with respect to the content of the silica;(A)_(a)(B)_(b)(C)_(c)(D)_(d)(R¹)_(e)SiO_((4-2a-b-c-d-e)/2)  (1) formula(1) being an average composition formula, wherein A is a divalentorganic group containing a sulfide group; B is a monovalent hydrocarbongroup having from 5 to 10 carbon atoms; C is a hydrolyzable group; D isan organic group containing a mercapto group; R¹ is a monovalenthydrocarbon group having from 1 to 4 carbon atoms; and a to e satisfythe relational expressions 0≦a<1, 0≦b<1, 0<c<3, 0<d<1, 0≦e<2, and0<2a+b+c+d+e<4, and at least one of a and b is not
 0. 2. The rubbercomposition for a tire tread according to claim 1, wherein a DBPabsorption number of the silica is at least 190 ml/100 g, and a ratio ofthe nitrogen adsorption specific surface area and the CTAB specificsurface area (nitrogen adsorption specific surface area/CTAB specificsurface area) is from 0.9 to 1.33.
 3. The rubber composition for a tiretread according to claim 1 further containing a terpene resin, whereinan amount of the terpene resin is from 1 to 30 parts by mass per 100parts by mass of the diene rubber, and the terpene resin is an aromaticmodified terpene resin having a softening point of from 60 to 150° C. 4.The rubber composition for a tire tread according to claim 1, wherein bis greater than 0 in formula (1).
 5. A pneumatic tire comprising tiretreads formed using the rubber composition for a tire tread described inclaim
 1. 6. The rubber composition for a tire tread according to claim 2further containing a terpene resin, wherein an amount of the terpeneresin is from 1 to 30 parts by mass per 100 parts by mass of the dienerubber, and the terpene resin is an aromatic modified terpene resinhaving a softening point of from 60 to 150° C.
 7. The rubber compositionfor a tire tread according to claim 2, wherein b is greater than 0 informula (1).
 8. The rubber composition for a tire tread according toclaim 3, wherein b is greater than 0 in formula (1).